Method of making a miniature inductive device



April 21; 1970 T, L. CRAIGE 3,507,039

METHOD OF MAKING MINIATURE INDUCTIVB DEVICE Filed Dec. 12/ 19s? 3 Sheets-Sheet 1 ATTORNEYS T. L. CRAIGE April 21, '1970 METHOD OF MAKING MINIATURE INDUCTIVE DEVICE Filed Dec. 12, 1967 3 Sheets-Sheet 2' INVENTOR 77/500025 4. 6364/55 v BY ATTORNEYS A ril 21, 1970 T.-| RA|GE 3,507,039

METHOD OF MAKING A MINIATURE INDUCTIVE DEVICE Filed Dec. 12, 1967 I a 3 Sheets-Sheet s II 1/] I l/ v rv i\ 20/\ I i INVENTOR 77/509025 t. CKA/G-f ATTO United States Patent O US. Cl. 29-605 7 Claims ABSTRACT OF THE DISCLOSURE A miniature inductive device capable of operating at high power levels and having a precisely defined air gap and maximum utilization of space allocated to coil windings.

BACKGROUND OF THE INVENTION In the art of fabricating miniature and sub-miniature inductive devices it is known to fabricate the coil bobbin of a suitable magnetic material which serves as the magnetic core of the device as well as the mechanical support for the coil windings. A thin layer of insulating coating on the bobbin provides adequate insulation between core and coil which is wound on the bobbin between flanges thereof. A cylindrical sleeve of magnetic material is utilized to complete the magnetic circuit with the main path consisting of bobbin, space between bobbin flange and cylindrical sleeve, cylindrical sleeve and the remaining space to the remaining bobbin flange.

The spacing between bobbin flanges and sleeve constitute an air gap which provides a major portion of the reluctance in the magnetic circuit. In certain applications it is desirable to minimize this air gap. In other applications an air gap of well defined width is desirable, such as where flux generated by direct current present in one or more of the windings might saturate the core, or in order to achieve maximum Q, for example. In all cases however, the gap ideally should be capable of being predetermined and achieveable in production.

In a prior art inductive device such as that shown in US. Patent No. 2,949,591, the cylindrical sleeve is formed from flat stock after which it is slipped over the wound bobbin. In such case a predetermined and/ or minimum space between bobbin flange and sleeve is not achievable within acceptable tolerances. Additionally, uniform annular spacing cannot be achieved, and finally the inherent memory in the sleeve material tends to open the sleeve or move it away from the bobbin flange. The degree of such movement is not predictable and of course this further renders the spacing not uniform. For example, in practice the minimum gap attainable is less than .001 inch for a cylindrical device having a height of 0.47 inch anda 0.33 inch outside diameter sleeve. The gap size cannot be precisely achieved and once the gap has been set it cannot be maintained and will change with time. i

In summary therefor with the present methods of fabrication a desired, predetermined gap cannot be achieved within acceptable tolerances and selection of completed devices having known reluctance is unsatisfactory as the gap may change with time and use in a random manner. All of this adds to the cost of fabrication and decreases'the desirability of the end product.

In addition the larger the gap, the higher the reluctance and the greater the number of turns and consequent heating. This increases the size and lowers the power capability of the device. Also the larger gap, the smaller the inductance. Further, termination of coil leads must be provided for. Often in prior art devices, the core material cross-section area has been reduced in order to provide a suitable path for the heavy termination leads. This too lowers inductance and power handling capacity.

SUMMARY OF THE INVENTION A miniature inductive device incorporating a wound bobbin within a cylindrical sleeve wherein the bobbin is substantially solid, having flanges with tapered inner wall surfaces and the sleeve is a continuous unslotted open-ended cylinder and where in assembly the bobbin is wound, inserted within the sleeve and expanded outwardly until the periphery of the bobbin flanges embrace the sleeve in a grinding action.

' Inductive devices arranged in accordance with the present invention may be employed as saturable reactors by employing an auxiliary saturating coil.

As further variations of the invention, more than two flanges may be employed to provide a multiple bobbin core, and the sleeve may be arranged either as a unitary cylinder for all bobbin sections of the core or separate sleeves may be employed for therseparate bobbin sections.

While the sleeve may preferably be placed without the rim of the flanges and externally thereof, it is also feasible under certain circumstances to have the sleeves extend between the walls of the flanges. In either construction the air gap is readily controllable, without employing expensive and slow methods of fabrication.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is an exploded perspective view of an inductive device constructed in accordance with the teaching of this invention;

FIGURE 2 is a top perspective view of one-method of assembly of the device shown in FIGURE-1 illustrating a wedge being applied for purposes of expanding the bobbin; 4

FIGURE 3 is a bottom perspective view of the expansion step illustrated in FIGURE 2;

FIGURE 4 is a top perspective view of the device shown in FIGURES 1-3 in the fully assembled condition; and

FIGURE 5 is an inverted longitudinal sectional view of the device shown in FIGURE 4.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION.

Referring now to the figures, the reference numeral 10 denotes a bobbin comprising a right circular cylinder 11 and two flanges 12 and 13 fabricated of suitable permeable magnetic material. The inner surfaces ofvfla'nges 12 and 13 are tapered-as best seen in FIGURE 5-so that the thickness of each of the flanges decreases uniformly as distance from the cylinder 11 increases.

The cylinder 11 is provided with a longitudinal radial slot 14 of small dimension for a purpose which will v be explained below. Flanges 12 and 13 are also provided outwardly beyond the outermost coil portion. r

is slightly greater than the width of slot 14.

The material is of sufficient thickness so that mechani cal rigidity is available. The flanges are circular and have perimeters substantially concentric with the. circular perimeter of cylinder 11. The radii of the flanges is preferably the same although in certain applications difiv ferent radii may be selected, and of sufficient dimension to allow each of the flange perimeters to extend 'outwardly beyond the outer perimeter of coil 17 wound upon cylinder 11. When multiple coils are wound on cylinder 11 the perimeters of the flanges should-extend In the present example the cylinder 11 as well as the iner surface of flanges '12 and 13 has been coated with a thin layer of insulation indicated by the numeral 18 in the figures toinsulate winding, 17 from the bobbin and the coil leads are allowed to project through slot 15 in flange 12 longitudinally for purposes of termination.

A cylindrical sleeve 20 is associated with the bobbin 10 and this sleeve may be of the same material of which the bobbin is fabricated although different material may be employed, if desired. Sleeve 20 should also be of a permeable magnetic material. The sleeve is cylindrical and has an inside diameter which prior to assembly is slightly greater than the outside diameter of the flanges 12 and 13. The sleeve is solid having no slots formed therein and in the embodiment shown is of approximately the same length or slightly longer than bobbin 10.

' Terminal board 21 is provided for terminating the coil 17. The ends of the coil are passed through slot 22 in the'board and wrapped around the upstanding ends of two of the electrical terminal members 23 of the plurality thereof which are partially embedded in board 21 as shown in FIGURE 1. Each of the members 23 is formed of a highly electrical conductive, rectangular cross-section material as is well known in the art.

The configuration of terminal board 21 is as shown for convenience in combining an integrated circuit package with the miniature inductive device of the subject invention and this particular configuration is by way of example only and terminal members of other configurations can be utilized.

After the lead ends have been wound about the ends of respective members 23, the ends are bent upon the board as shown in FIGURE 2 and solder as indicated by the numeral 24 is applied, completing the electrical connection and insuring mechanical stability. Means other than solder may be utilized to make the joint.

In usual practice a layer of cement 25 is applied between board 21 and flange 12 prior to folding and soldering of the ends of members 23 and a wrap of insulation tape 26 is applied around coil 17 as shown in FIGURE prior to insertion within sleeve 20.

A suitable cement or resin having high insulation, moisture resistance and bonding properties is then positioned at zones indicated by the numerals 27, 28, and 29 in the figures and the assembly completed by disposition of insulating boards 30 and '31 at the ends of the device.

As heretofore stated, it is of utmost importance in certain applications to minimize the distance between the perimeter of flange 12 and flange 13 respectively with the adjacent portion of the inner surface of sleeve 20. In other applications it is of equal importance to provide a spacing between flange and sleeve which is precisely predetermined and uniform throughout the flange circumference.

In order to achieve this precisely defined predetermined spacing whether it be minimum or of a fixed value the bobbin in the subject application, after winding and insertion within sleeve 20, is expanded radially outwardly. This can be accomplished after insertion within the sleeve 20 and prior to positioning of terminal board 21 or it can be accomplished after positioning of terminal board 21 by provision of a central aperture 32 in terminal board 21.

The radial expansion of the bobbin may be accomplished in any suitable manner. In FIGURES 2 and 3 a specific method of expansion is disclosed; however, it should be understood that this specific method is for purposes of explanation only and the inventive concept disclosed herein embraces all methods of expansion of the bobbin.

As shown in FIGURES 2 and 3 wedges 33 and 34 are respectively applied to the slot- 14 in the bobbin at the top and bottom ends thereof. By the application of force to the wedges in the directions illustrated by the arrows in FIGURES 2 and 3 the wedges will be driven longitudinally within slot 14 and through wedging action cause the bobbin to expand radially outwardly.

Upon expansion radially outwardly of the bobbin as a result of this wedging action, the flanges 12 and 13 are moved toward the adjacent or facing surfaces of sleeve 20 and upon application of sufiicient force are ground therein in a grinding action forming the adjacent surface of the sleeve in a conforming surface so that a minimum air gap can be achieved. The achievable air gap is minimized and substantially uniform throughout the periphery of the respective flange. Additionally, with a properly designed device a known application of force can result in a predetermined gap as measured by the reluctance in the magnetic circuit and the ultimate performance of the unit.

Although the exact and complete reason for the improved performance of the design of the subject invention is not completely known, it is believed that the bobbin is permanently deformed and given a new neutral or unstressed position and the inherent memory in the sleeve tends to draw the sleeve toward the bobbin after forces which expand the bobbin and sleeve outwardly during the wedging action are removed. Also, in the subject design the expanding out of the bobbin tends to decrease the tensile force built up in the coil during winding of the bobbin and therefore increasing reliability of the device.

It has been found that in certain applications the grinding action of flange perimeter with respect to the facing surface of sleeve 20 is not necessary but rather a precisely predetermined movement or expansion of the bobbin flanges toward the sleeve is desirable in view of carefully setting the reluctance in the magnetic path. In such case the deforming of the bobbin determines the air gap.

With the utilization of the bobbin expanding operation, the spacing between bobbin flange and facing sleeve surface can be minimized, can be predetermined, and can be made substantially uniform or annular. In addition, the forces tending to change the gap distance by urging the flanges radially inwardly and/or the sleeve radially outwardly are substantially eliminated so that the achievable magnetic path reluctance of the particular device will remain substantially constant with time.

In the prior art device, illustrated in U.S. Patent No. 2,949,591, wherein the sleeve is formed from flat stock a cylindrical casing of uniform diameter throughout its height cannot be achieved in practice. With the subject invention the expanding out of the bobbin toward the cylindrical sleevewithout a gapwill allow for a configuration wherein one of the flanges can be made with a larger diameter than the remaining flange so as to allow the periphery of the larger flange to embrace the sleeve while the remaining flange is spaced therefrom. This is because the sleeve can be formed with a substantially uniform diameter throughout its height.

Under certain circumstances, it may be desirable to achieve the gap between flange and sleeve by providing an insulating material on the flange periphery, which, upon encountering the sleeve during the expanding operation, will prevent further expansion and determine the gap.

In practice, the gap is set at a determined spacing slightly more than the gap which it is anticipated will be required for a specified low frequency response. The inductance which controls the low frequency response of the device to electrical signals is then measured as the gap is decreased until the desired inductance has been obtained. A very high degree of accuracy can be obtained. For example the gap can be set at .0001 inch for a cylindrical device having a height of .250 inch and a .250 inch outside diameter sleeve. A low end frequency response of 300 Hz. can be obtained.

The small dimensions of the slots in the bobbin minimizes the reduction of the core and maximizes inductance and power handling capability of the device. Also,since the slot14 is utilized in the embodiment disclosed herein for purposes of radially expanding the bobbin through the application of wedges, upon removal of the wedges, additional core material can be provided by insertion of a slug of the magnetic material in slot 14 diminishing the slot.

In particular applications one end flange only need be slotted to allow passage of the termination leads.

It should also be noted that When desired one end only of the bobbin core need be slotted for wedging and expanding of the bobbin. In such application the deformation of the bobbin at one end is used to position the bobbin flange at the end with respect to the sleeve in order to determine the air gap.

In addition to the above the small size and the power handling capability of the device is enhanced by the provision of maximum number and regular configuration coil windings within the space between the flanges. The tapered inner surfaces of the flanges 12 and 13 provide the ability to wind the coil in an even lay of adjacent turns. This configuration allows the utilization of additional space for the wire and eliminates the additional nonuseful turns resulting from irregular or cross turns.

I claim:

1. That method of fabricating an inductive device including providing a flanged bobbin formed out of magnetic material, winding an electrically conductive wire about said bobbin between flanges thereof, providing a magnetic casing and inserting said bobbin therein, and thereafter expanding said bobbin radially outwardly in the direction of said casing, and securing said bobbin within said casing against relative movement with respect to each other.

2. That method of fabricating an inductive device including providing a flanged bobbin formed out of magnetic material, winding an electrically conductive wire about said bobbin between flanges thereof, providing a magnetic casing and inserting said bobbin therein, said magnetic casing having surfaces with diameters slightly greater than the outside diameters of respective facing flanges, and thereafter expanding said bobbin radially outwardly to bring the periphery of said flanges into adjacency with the facing surfaces of said casing.

3. That method of fabricating an inductive device including providing a flanged bobbin formed out of magnetic material, winding an electrically conductive wire about said bobbin between flanges thereof, providing a magnetic casing and inserting said bobbin therein, and thereafter expanding said bobbin radially outwardly to decrease tensile stresses developed in said wire during winding about said bobbin, and securing said bobbin within said casing against relative movement with respect to each other.

4. That method of fabricating an inductive device including providing a flanged bobbin formed out of magnetic material, winding an electrically conductive wire about said bobbin between flanges thereof, providing a magnetic casing and inserting said bobbin therein, said casing having surfaces with diameters slightly greater than the'outside diameters of respective facing flanges, and thereafter expanding said bobbin radially outwardly to enable the periphery of at least one of said flanges to embrace surfaces of said casing in agrinding action.

7 5. That method of fabricating an inductive device including providing a flanged bobbin formed out of magnetic material, providing said bobbin with a longitudinally extending radial slot, winding an electrically conductive wire about said bobbin between flanges thereof, providing a cylindrical magnetic casing and inserting said bobbin therein, inserting wedges in said slots at either end thereof, applying longitudinally directed force to said wedges thereby expanding said bobbin radially outwardly in the direction of said casing, and thereafter removing said wedges.

6. That method of fabricating an inductive device as set forth in claim 5 wherein a slug of magnetic material is inserted in the slot after removal of the wedges.

7. That method of fabricating an inductive device in accordance with claim 1 wherein insulating material is provided upon the peripheries of said flanges to arrest movement of said flanges toward said casing during expanding of said bobbin.

References Cited UNITED STATES PATENTS 2,949,591 8/1960 Craige 336-83 2,976,907 3/1961 Harvey et al. 3,381,251 4/1968 Fuller 336-212 X CHARLIE T. MOON, Primary Examiner C. E. HALL, Assistant Examiner US. Cl. X.R. 

